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A-Z of Chest Radiology

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 (ISBN-13: 9780511332289)

A–Z of Chest Radiology

Cambridge University Press
9780521691482 - A–Z of Chest Radiology - by Andrew Planner, Mangerira C. Uthappa and Rakesh R. Misra
Excerpt


FUNDAMENTALS OF CXR INTERPRETATION – ‘THE BASICS’   PART I

• QUALITY ASSESSMENT1
• PATIENT-DEPENDENT FACTORS3
• REVIEW OF IMPORTANT ANATOMY7
• LUNGS AND PLEURA11
• DIAPHRAGMS13
• BONES AND SOFT TISSUES14
• A BRIEF LOOK AT THE LATERAL CXR16

When interpreting a CXR it is important to make an assessment of whether the x-ray is of diagnostic quality. In order to facilitate this, first pay attention to two radiographic parameters prior to checking for pathology; namely the quality of the film and patient-dependent factors. A suboptimal x-ray can mask or even mimic underlying disease.

Quality assessment

Is the film correctly labelled?

This may seem like an obvious statement to make. However, errors do occur and those relating to labelling of the radiograph are the most common.

What to check for?

  • Does the x-ray belong to the correct patient? Check the patient’s name on the film.
  • Have the left and right side markers been labelled correctly, or does the patient really have dextrocardia?
  • Lastly has the projection of the radiograph (PA vs. AP) been documented?

Assessment of exposure quality

Is the film penetrated enough?

  • On a high quality radiograph, the vertebral bodies should just be visible through the heart.
  • If the vertebral bodies are not visible, then an insufficient number of x-ray photons have passed through the patient to reach the x-ray film. As a result the film will look ‘whiter’ leading to potential ‘overcalling’ of pathology.
  • Similarly, if the film appears too ‘black’, then too many photons have resulted in overexposure of the x-ray film. This ‘blackness’ results in pathology being less conspicuous and may lead to ‘undercalling’.
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The effect of varied exposure on the quality of the final image.

Is the film PA or AP

  • Most CXRs are taken in a PA position; that is, the patient stands in front of the x-ray film cassette with their chest against the cassette and their back to the radiographer. The x-ray beam passes through the patient from back to front (i.e. PA) onto the film. The heart and mediastinum are thus closest to the film and therefore not magnified.
  • When an x-ray is taken in an AP position, such as when the patient is unwell in bed, the heart and mediastinum are distant from the cassette and are therefore subject to x-ray magnification. As a result it is very difficult to make an accurate assessment of the cardiomediastinal contour on an AP film.
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The cardiomediastinal contour is significantly magnified on this AP film. This needs to be appreciated and not overcalled.

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On the PA film, taken only an hour later, the mediastinum appears normal.

Patient-dependent factors

Assessment of patient rotation

  • Identifying patient rotation is important. Patient rotation may result in the normal thoracic anatomy becoming distorted; cardiomediastinal structures, lung parenchyma and the bones and soft tissues may all

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    A well centred x-ray. Medial ends of clavicles are equidistant from the spinous process.

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    This patient is rotated to the left. Note the spinous process is close to the right clavicle and the left lung is ‘blacker’ than the right, due to the rotation.

    appear more, or less, conspicuous. To the uninitiated, failure to appreciate this could easily lead to ‘overcalling’ pathology.

  • On a high-quality CXR, the medial ends of both clavicles should be equidistant from the spinous process of the vertebral body projected between the clavicles. If this is not the case then the patient is rotated, either to the left or to the right.

  • If there is rotation, the side to which the patient is rotated is assessed by comparing the densities of the two hemi-thoraces. The increase in blackness of one hemi-thorax is always on the side to which the patient is rotated, irrespective of whether the CXR has been taken PA or AP.

Assessment of adequacy of inspiratory effort

Ensuring the patient has made an adequate inspiratory effort is important in the initial assessment of the CXR.

  • Assessment of inspiratory adequacy is a simple process.
  • It is ascertained by counting either the number of visible anterior or posterior ribs.
  • If six complete anterior or ten posterior ribs are visible then the patient has taken an adequate inspiratory effort.
  • Conversely, fewer than six anterior ribs implies a poor inspiratory effort and more than six anterior ribs implies hyper-expanded lungs.
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Six complete anterior ribs (and ten posterior ribs) are clearly visible.

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An example of poor inspiratory effort. Only four complete anterior ribs are visible. This results in several spurious findings: cardiomegaly, a mass at the aortic arch and patchy opacification in both lower zones.

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Same patient following an adequate inspiratory effort. The CXR now appears normal.

  • If a poor inspiratory effort is made or if the CXR is taken in expiration, then several potentially spurious findings can result:
    • apparent cardiomegaly
    • apparent hilar abnormalities
    • apparent mediastinal contour abnormalities
    • the lung parenchyma tends to appear of increased density, i.e. ‘white lung’.
  • Needless to say any of these factors can lead to CXR misinterpretation.

Review of important anatomy

Heart and mediastinum

Assessment of heart size

  • The cardiothoracic ratio should be less than 0.5.
  • i.e. A/B < 0.5.
  • A cardiothoracic ratio of greater than 0.5 (in a good quality film) suggests cardiomegaly.
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Assessment of cardiomediastinal contour

  • Right side:
    • SVC
    • RA
  • Anterior aspect:
    • RV
  • Cardiac apex:
    • LV
  • Left side:
    • LV
    • Left atrial appendage
    • Pulmonary trunk
    • Aortic arch.
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Assessment of hilar regions

  • Both hilar should be concave. This results from the superior pulmonary vein crossing the lower lobe pulmonary artery. The point of intersection is known as the hilar point (HP).
  • Both hilar should be of similar density.
  • The left hilum is usually superior to the right by up to 1 cm.
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Assessment of the trachea

  • The trachea is placed usually just to the right of the midline, but can be pathologically pushed or pulled to either side, providing indirect support for an underlying abnormality.
  • The right wall of the trachea should be clearly seen as the so-called right para-tracheal stripe.
  • The para-tracheal stripe is visible by virtue of the silhouette sign: air within the tracheal lumen and adjacent right lung apex outline the soft-tissue-density tracheal wall.
  • Loss or thickening of the para-tracheal stripe intimates adjacent pathology.
  • The trachea is shown in its normal position, just to the right of centre. The right para-tracheal stripe is clearly seen.
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Evaluation of mediastinal compartments

It is useful to consider the contents of the mediastinum as belonging to three compartments:

  • Anterior mediastinum: anterior to the pericardium and trachea.
  • Middle mediastinum: between the anterior and posterior mediastinum.
  • Posterior mediastinum: posterior to the pericardial surface.



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